Hi Bud,
Sounds very similar to my setup. The main difference is I'm using three 4-el
SteppIRs instead of three KT36XAs. I originally had a 40-2CD at 110' being
turned by a T2X, but replaced it with a Cal-AV 2D-40A, which is a *much*
larger antenna. That required replacing the T2X with an M2 Orion. There are
some pictures of my tower on my website at www.wc1m.com. I hope to add more
pics of the installation if I can ever find the time to process the photos
and write the descriptions.
I used a full-size section on the bottom with a pier-pin. Sounds like you're
going to sink the short base in the concert. For a larger tower with a heavy
load, I prefer the pier-pin configuration. A little more difficult to
install (cf, what happened to W2GD), but worth it in my opinion. The key is
to use a safe temporary guy system.
Are you using 11 sections (110' of tower) or fewer sections and a longer
mast? I used 11 55G sections and placed the 2-el 40m beam just above the
thrust bearing in the top plate. The very short moment arm between the
antenna and thrust bearing allowed me to use a 12-foot aluminum mast with
1/4" wall (about 6 feet in the tower and 6 feet above the top plate.) So
far, no problems with the mast bending or anything. The relatively light
aluminum mast was easy to install, and I don't have to worry about
corrosion. I was able to combine truck freight with an identical aluminum
tube used for my gin pole, so the cost was quite reasonable.
The extra section above the top set of guys makes a lot of sense to me. You
need at least 10 feet of separation between the XM-240 and top tribander,
preferably more, and it's a heck of a lot easier to climb 10' of tower to
maintain the 40m antenna than deal with raising and lowering a long mast. I
don't know the engineering, but would expect 10' of 55G to be at least as
strong as 10' of mast, probably a lot stronger.
Not clear from the TIC ring positions where you're going to place the guys.
For a 110' 55G tower, Rohn specifies guy positions of 105', 70' and 35'. But
since my top guy (like yours) could be no higher than 95', I used the guy
positions for 100' of 55G, which are 95', 63' and 33'. The TIC rings are at
96', 64' and 34', each 1 foot above the guy brackets. I would have used the
same guy positions for a 100' tower with 5' of tower and 10' of mast above
the top guy, so it made sense to use them with 15' of tower above the top
guy.
Another advantage to the guy positions I used is that they provide 7' more
vertical spacing between the middle antenna and the top guys, which are the
ones you have to worry about. The turning radius of the 4-el SteppIR is
about 2.5' longer than the KT36XA, so I needed all the clearance I could
get. Also, the tower is on a steep hill and I was trying to keep the guy
angles as close to Rohn spec as possible.
Comparing 96'/64'/34' versus 95'/70'/35' in HFTA at my location shows that
the latter is slightly better, but only by .4 dB on 20m, .2 dB on 15m and .3
dB on 10m. For me, the clearance issue with the middle antenna outweighed
that small gain. You can imagine my relief when I turned the middle antenna
for the first time and there was enough clearance.
Speaking of clearance, if you use all-EHS guys, be sure to take the catenary
into account when computing clearance for the middle antenna. I used
Phillystran guys with EHS leaders at the bottom, so the catenary wasn't an
issue.
Hope this helps.
73, Dick WC1M
> -----Original Message-----
> From: Bud Trench [mailto:aa3b.bud@gmail.com]
> Sent: Wednesday, July 30, 2008 9:53 PM
> To: towertalk@contesting.com
> Subject: [TowerTalk] Please comment on a Tower Design
>
> I'd appreciate comments or recommendations on the following proposed
> tower configuration. Many thanks! 73 Bud AA3B
>
> Location: Berks County, PA - 70 mph zone
>
> Tower: 110' 55G, 110 mph, no ice (55G110D110), SB55G short base
>
> Cushcraft XM240 @ 110' on a 2" O.D. High carbon steel mast, TB3 thrust
> bearing, T-2X rotor, AS455 accessory shelf
>
> Qty 3 M2 KT36XA on Tic Rings @ 95'/70'/35'
>
> Stack Match
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